1. Field of the Invention
The present invention relates to an optical communications system applicable to long-distance communications such as underseas cable communications, etc.
2. Description of the Related Art
Recently, optical communications systems have been widely developed to realize large-capacity and high-speed communications systems. Especially when a large volume of information is to be simultaneously transmitted, an optical wave-length multiplexing system is highly evaluated and is studied for practical use in the near future. In the optical wave-length multiplexing system, an optical signal which carries information and have a plurality of wavelengths is wavelength-multiplexed for transmission. An optical signal of each wave length corresponds to at least one communications channel. In the optical wavelength multiplexing system applicable to the long-distance communications such as underseas cable communications, an optical add-drop system is under development in which an optical signal having a specific wavelength or an optical signal along a specific channel among optical signals wavelength-multiplexed in the communications line is branched to transmit an optical signal along a channel branched to a terminal station, and the optical signal transmitted from the terminal station with the same wavelength as the branched channel is combined again to the optical signal transmitted through the original transmission line for transmission to the terminating station.
FIGS. 1A through 1F show the conventional optical add-drop system and the problem with the system.
FIG. 1A is a block diagram showing the entire configuration of the optical add-drop system. The basic configuration in the optical add-drop system has a terminal station A as a transmitting station for transmitting an optical-wavelength multiplexed optical signal, a terminal station C as a receiving station for receiving a signal from the terminal station A, a branching unit 1100 for branching or combining an optical signal of a specific wavelength in the optical signals from the terminal station A, and. a terminal station B for receiving the optical signal branched by the branching unit 1100, and transmitting new information with an optical signal having the same wavelength as the received optical signal. Normally in the underseas cable communications, the branching unit 1100 is mounted underseas to transmit optical signals to, for example, the terminal stations A, B, and C provided in different nations. Typically, the distance between the terminal stations A and C is approximately 3,000 km, and the branching unit 1100 is provided around the central point between these stations. Since the intensity of an optical signal is attenuated when the optical signal is transmitted for a long distance, the transmission lines between the terminal station A and the branching unit 1100, between the terminal station B and the branching unit 1100, and between the terminal station C and the branching unit 1100 have a plurality of optical amplifiers 1101, 1102, and 1103 respectively. FIG. 1A shows the optical amplifiers 1101, 1102, and 1103 apiece for respective transmission lines for a simple illustration, but there are actually much more optical amplifiers for each transmission line. Normally, each of the optical amplifiers 1101, 1102, and 1103 has an automatic output level control circuit (ALC circuit) to keep the output level of each of the optical amplifiers 1101, 1102, and 1103 constant so that the optical signal can be constantly amplified to a specific output level.
FIG. 1A shows the transmission line for one-way communications. Actually, the circuit is designed to establish two-way communications, that is, up-line and down-line communications.
FIGS. 1B through 1F show an optical signal and its problem in each transmission line.
FIG. 1B shows the optical signal at point A in FIG. 1A. In the case shown in FIG. 1B, optical signals having four different wavelengths are wavelength-multiplexed and transmitted from the terminal station A. The mound under each optical signal is called an amplified spontaneous emission (ASE) noise. It is produced when a noise superposed to an optical signal is amplified with the optical signal by an optical amplifier. The characteristics of the operations of the optical communications system depend on the S/N ratio of the optical signal to the ASE.
In the branching unit 1100, the optical signal having a wavelength .lambda..sub.1 is branched and transmitted to the terminal station B, and an optical signal having the wavelength .lambda..sub.1 is transmitted from the terminal station B to the terminal station C.
An optical signal having a wavelength other than wavelength .lambda..sub.1 in the signal (FIG. 1B) transmitted from the terminal station A is not branched by the branching unit 1100, but is transmitted as is to the terminal station C. The terminal station B receives the optical signal having wavelength .lambda..sub.1 and transmits an optical signal having the same wavelength .lambda..sub.1. FIG. 1C shows the state at point B of the signal transmitted from the terminal station B and amplified by the optical amplifier 1102. The branching unit 1100 combines the optical signal having wavelength .lambda..sub.1 transmitted from the terminal station B with the light having wavelength .lambda..sub.2 through .lambda..sub.4, and transmits the result to the terminal station C.
FIG. 1D shows the state at point C of the optical signal from the terminal station B which is combined by the branching unit 1100 and amplified by an optical amplifier 1103. FIGS. 1C and 1D show the case where the power level of an optical signal is equal to that of each other when the optical signal from the terminal station B is combined with the optical signal from the terminal station A. In this case, an optical signal having any wavelength indicates the same S/N ratio to the ASE noise as shown in FIG. 1D.
FIG. 1E also shows the state of the optical signal at point B. In this case, the power level of the optical signal from the terminal station B is high. When the power level of the optical signal from the terminal station B is high, the state of the optical signal at point C after being combined by the branching unit 1100 and being amplified by the optical amplifier 1103, becomes as shown in FIG. 1F. Therefore, although the S/N ratio of wavelength .lambda..sub.1 is high, because the operation characteristics of the optical communications system are based on the lower S/N ratio, when the S/N ratios of the other wavelengths are low, the system is recognized as poor in operation characteristics.
FIGS. 2A, 2B, 3A, and 3B show the operation of the optical amplifier and the S/N ratio.
In this example, the two optical signals having different wavelengths are multiplexed, and an optical signal of a total of 0 dBm power is input to the optical amplifier. The optical amplifier includes an automatic output level control circuit having a gain of 10 dB and an optical output is limited to 10 dBm. The state of the optical signal at the input terminal is -3 dBm each for the power of the optical signals of two wavelengths, a total of 0 dBm as shown in FIG. 2A. FIG. 2B shows the output when such optical signals are input to the optical amplifier. That is, the optical signal of each wavelength is amplified, and the power of each optical signal is +7 dBm with a total power of the output light indicating +10 dBm. On the other hand, the ASE noise is also amplified, and the S/N ratio to the ASE noise of each optical signal is 30 dB. Therefore, the operation characteristic of the optical amplifier indicates the S/N ratio of 30 dB.
FIGS. 3A and 3B show the case where an input optical signal is multiplexed with an optical signal having a different power level. The characteristic of the optical amplifier is the same as that of the optical amplifier shown in FIGS. 2A and 2B. However, as shown in FIG. 3A, a total power of the optical signals having two different wavelengths is 0 dBm with the power level of one optical signal indicating -1.5 dBm while the other optical signal indicating 4.5 dBm. There is 3 dB difference between the power levels. If such optical signals are input, the output is obtained as shown in FIG. 3B. That is, the higher power level of the optical signal between the two input signals is +8.5 dBm while the lower power level of the optical signal is +5.5 dBm because the optical signal having each wavelength is amplified such that the total power level of the output signals can be the above described value, that is, the output of the optical amplifier is fixed to +10 dBm. At this time, the ASE noise is amplified and the S/N ratios are different between the wavelengths. That is, the S/N ratio of the wavelength indicating the higher power level is an acceptable value while the S/N ratio of the wavelength indicating the lower power level is relatively undesired. Since the operation characteristic of the optical amplifier is evaluated by the undesired S/N ratio, the performance of the optical amplifier is considered to be poor.
In the optical add-drop system as described above by referring to FIG. 1A, a lot of optical amplifiers are inserted between the terminal station and the branching unit. In the branching unit, an independently generated optical signal from the terminal station A is combined with an optical signal from the terminal station B, and amplified by the optical amplifier. The optical signals of respective wavelengths from the terminal stations A and B may not match in power when they are combined because of the transmission distance and the difference in output. Furthermore, the power level of the optical signal may not be controlled just as designed even if the system has been formed by carefully computing the output power and the attenuation of the optical signal in the designing step. In this case, there arises a difference in S/N ratio between the optical signal having a lower power level and the optical signal having a higher power level after the amplification through the optical amplifier as described by referring to FIGS. 2A, 2B, 3A, and 3B. The operation characteristic of the system is evaluated by the S/N ratio of the optical signal having the lower power level, that is, the undesired S/N ratio.
When the power level of the optical signal from a branch station is different from that of the optical signal from the transmitting station, the evaluation is made based on the lower S/N ratio indicating the transmission characteristic of the optical signal, thereby considering the system to be poor in performance.